The effect of myocardial hypertrophy and the fundamental state of hypertrophied myocardium on passive stiffness of the muscle will be examined. Right ventricular papillary muscles from normal cats and pulmonary artery banded cats will be studied in an isolated muscle bath containing modified Kreb's solution at 30 degrees C stimulated at a frequency of 12/min. Hypertrophied muscles with normal contractile function and hypertrophied muscles with depressed contractile function can be obtained by sacrificing the animals at selected periods after banding. Passive stiffness will be assessed by determining the instantaneous stress-natural strain relations determined from the resting length-tension relations of isometrically contracting muscles. In addition, similar stress-strain relations will be determined from the resting-length tension relations of slowly stretched (0.2 mm/min.) non-contracting muscle. The effect of acetylstrophanthidin on passive stiffness of normal, non-failing hypertrophied and failing hypertrophied myocardium will be determined in a similar manner. Stress-strain relations of papillary muscles from normal and adult banded cats will be examined under control conditions, at near maximal inotropic effect produced by the addition of increments of acetylstrophantidin and after toxic manifestations of acetylstrophanthidin have appeared.